1. Field of the Invention
The present invention relates to an extruder die head, preferably a blown film head, comprising a central annular channel, which is provided with an annular outlet die and into whose outer limiting wall empty annular slits, which feed a polymer melt and which constitute the smaller diameter openings of truncated channels, formed between the internal and external shells of stacked, conical insert members.
2. Related Art
EP 0 568 544 B1 discloses an extruder head for extruding multi-layered thermoplastic pipes of the kind described above, whose central annular channel is defined by a central mandrel, whose shell forms the inside wall of the channel, and by stacked conical insert members, which enclose the channel and whose inside openings form the outer wall of the central annular channel. In this prior art extruder heads a truncated-channel is formed between two bell shaped or conical insert members, which are provided with a radial feed borehole for the polymer melt, For each tubular layer of polymer melt to be extruded there are two conical insert members, which are stacked one over the other. The space between these insert members forms the annular slits. The insert members are held together by these clamping cover parts, which are connected together with tightening screws. The prior art extruder die head exhibits a significant overall length, when multilayered pipes are to be produced. However, such long extruder die heads exhibit the drawback that the large height of the extruder die head results in long flow paths for the polymer melt. These paths in turn result in high Theological stress at the melt interfaces, a state that can lead to unstable flow behaviour. In particular, the long flow paths are a problem with polymer melts that cannot tolerate heating for a prolonged period of time. Such polymer melts decompose and become brittle when heated for long periods.
Therefore, the object of the invention is to provide an extruder die head of the class described in the introductory part. This extruder die head facilitates the extrusion of plastic tubes with arbitrary layers, but is characterized by a significantly shorter construction height.
The invention solves this problem in that the annular slits, feeding the polymer melts, also empty into the inside wall of the central annular channel. The annular slits are the smaller diameter openings of truncated channels, formed between the internal and external shells of stacked conical internal insert members.
It is also easy to build the extruder die head of the invention in modules. A number of conical insert members are stacked until the desired number of layers of the extruded tube is reached. While maintaining the same number of conical feed channels, thus the same number of extruded tubular layers, it is possible to make the extruder die head of the invention half the construction length of the prior art extruder die head, because, based on the length of the conical insert members, two annular slits that feed the polymer melt can empty into the central annular channel. The significantly reduced axial length of the extruder die head of the invention results in an improved flow pattern of the melt that is fed in and less heat stress on the melt, because the melt spends correspondingly less time in the extruder die head.
The overall length of the extruder die head of the invention can be further reduced in that the internal and external shells of each insert member define the truncated channels for feeding the polymer melts into the central annular channel. In contrast to the prior art extruder die head, this design reduces the size of the conical insert members to half of their former size so that the overall length is correspondingly shortened.
The internal and external annular slits, which empty into the central annular channel, can lie in the same radial planes, Of course, it would also be possible to move the annular slits axially.
Preferably the internal and external shells of the conical insert members are two counter rotating spiral channels, whose depth tapers off in the direction of the opening. This design off the channels, wherein the melt overflows the channels in the axial direction, is well known.